1. Field of the Invention
This invention relates to a method for preparing corundum particles by a hydrothermal process, and more particularly to a method of efficiently preparing corundum particles in almost spherical and polyhedric forms which are excellent in crystallinity and high in purity.
2. Description of the Prior Art
Corundum particles which are known to be industrially useful as an abrasive material, electric insulator, material for fusion spraying, sintered material, various kinds of fillers and the like have been heretofore produced by finely powdering fused corundum blocks obtained from Bayer alumina or bauxite as a starting material. However, this production process is disadvantageous in that the corundum particles obtained by the process are relatively poor in crystallinity and purity and are irregular in shape. In order to overcome the above disadvantage, there have been proposed several processes of preparing single crystal particles of corundum, for example, in Japanese Patent Publication Nos. 7750/1962 and 13465/1964, in which processes alumina or alumina hydrate with low crystallinity is subjected to a hydrothermal treatment to obtain particles of hexagonal platy or columnar corundum single crystals. However, these processes also have the disadvantage that the size of the corundum particles cannot be suitably controlled even by properly selecting the treating conditions falling within the range described in the above patent specifications, such as particle size of the starting material, pressure, temperature, etc. Thus, these processes are not necessarily suitable for producing corundum particles with a desired size in an efficient manner. Further, as will be understood from the Examples of the foregoing patent specification, the production of corundum particles by the hydrothermal treatment of an alumina hydrate at 450.degree. C. or less undesirably requires a high pressure of generally above 500 kg/cm.sup.2, preferably above 1000 kg/cm.sup.2. This in turn requires costly equipment so as to realize the industrial production of corundum particles under such severe pressure conditions. In addition, the corundum particles obtained by these processes are substantially composed of hexagonal platy or columnar crystals and contain almost no spherically polyhedric particles.